Add '-fopenmp'

[ysk24ok]

If torch==1.6.0 is installed, following warning is raised when I try to build a wheel.

$ python setup.py bdist_wheel
gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -I/hdd/nishioka/warp-ctc/include -I/usr/local/cuda-10.2/include -I/opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include -I/opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include/torch/csrc/api/include -I/opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include/TH -I/opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include/THC -I/usr/local/cuda/include -I/opt/pyenv/versions/3.8.5/include/python3.8 -c src/binding.cpp -o build/temp.linux-x86_64-3.8/src/binding.o -std=c++14 -fPIC -DWARPCTC_ENABLE_GPU -DTORCH_API_INCLUDE_EXTENSION_H -DTORCH_EXTENSION_NAME=_warp_ctc -D_GLIBCXX_USE_CXX11_ABI=0
In file included from /opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include/ATen/Parallel.h:149,
                 from /opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include/torch/csrc/api/include/torch/utils.h:3,
                 from /opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include/torch/csrc/api/include/torch/nn/cloneable.h:5,
                 from /opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include/torch/csrc/api/include/torch/nn.h:3,
                 from /opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include/torch/csrc/api/include/torch/all.h:7,
                 from /opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include/torch/extension.h:4,
                 from src/binding.cpp:6:
/opt/pyenv/versions/3.8.5/lib/python3.8/site-packages/torch/include/ATen/ParallelOpenMP.h:84: warning: ignoring #pragma omp parallel [-Wunknown-pragmas]
 #pragma omp parallel for if ((end - begin) >= grain_size)

This is because '-fopenmp' is missing when building src/binding.cpp, so I've added it.